In particular, the method is applicable to chiral smectic phase ferroelectric liquid crystal screens functioning by transmission, electroluminescent screens functioning by emission and more particularly to micropoint screens.
For the sake of convenience, the following description is made by referring principally to the case of ferroelectric liquid crystal display screens, but this of course is not restrictive and in particular persons of ordinary skill in this field could easily readily transpose the means thus described to any other type of screen, such as, for example, micropoint emitting screens.
For the same reasons, there now follows a description of the black or white states of each pixel by reference to what effectively is present with screens that function by transmission. In the case of screens that function by emittion, as with micropoint photoluminescent screens, the description of the black and white states of each pixel is by reference, respectively, to the maximum emission state and the absence of any emission state of a specific pixel.
The invention is particularly applicable to optoelectronic techniques, the field of television and the binary (or analog) display of complex images, as well as to the display of alphanumeric characters. Firstly, of course it concerns display in black and white, but also the display of color images, as shall be seen subsequently.
Although the invention is applicable to any type of tilted smectic chiral liquid crystals it refers more particularly to chiral smectic phase liquid crystals.
As regards the details of the present invention, consideration shall be given to the constitution of ferroelectric liquid crystal display screens recognised by persons of ordinary skill, said screens being described in detail in the documents EP-A-0092181 and EP-A-0032362. The main characteristics of these shall not be described here in detail, except to mention that a liquid crystal display screen generally comprises two crossed polarizers between which is disposed a cell containing the crystal and provided on its walls with two matrix systems of right angle i line conductors and j column conductors, the various pixels of the image being defined by the crossings of these superimposed conductors. It is fundamental for an understanding of the present invention, that the ferroelectric liquid crystals used until now mostly have a bistable state allowing for a switching from black to white (or vice versa) under the control of the electric field applied at each point of the crystal from the potential difference V established by the line conductor and column conductor referring to this pixel. The switching from one state to another is effective when the product Vt of the voltage V applied at each point by the time t during which it is applied exceeds a threshold value (Vt).sub.threshold.
The problem which the present invention resolves is of writing a certain number of shades or levels of grey on a black and white image, it being understood, as has just been mentioned, that the cells of the ferroelectric liquid crystals display only makes it possible to embody shades of grey from a mixture of opaque (black) surfaces or totally transmitting (white) surfaces.
The problem of inscribing by means of such a screen a certain number of intermediate shades of grey between black and white is one already investigated by researchers and this point as regards this field has been fully discussed firstly by the document entitled CONFERENCE RECORDS OF THE 1985 INTERNATIONAL DISPLAY RESEARCH CONFERENCE, San Diego, Calif., Oct. 15-17, 1985, pages 215 to 220, and secondly the document EP-A-0 219 479 by the same authors.
The first of these methods consists of embodying a certain number of states, each of which has its own switching threshold. This is embodied, either by using ferroelectric liquid crystals in several stable states, a technique which is just starting to be employed, or by embodying on a pixel several ranges of different thicknesses. In the event, those technologies using solutions of this type are extremely difficult to implement.
A further solution, also specified by the preceding documents, consists of collectively associating several adjacent sub-pixels with a different surface area. Thus, by combining the transmission coefficient proportional to the surface of each of them with black or white display, it is possible to obtain a higher number of levels of grey. By way of example, if each pixel is subdivided into four sub-pixels whose surfaces are in the ratios 1, 2, 4 and 8, a binary display base is thus constituted which, via a combination of the preceding surfaces and the two black and white states, makes it possible to display 16 different levels of grey. The possible number of the levels of grey to be thus obtained is 2.sup.n if each pixel comprises n sub-pixels. The result obtained is therefore relatively modest if one considers the resultant increase in complexity of the screen and the fact that a significantly higher number of levels of grey is required so as to be able to display a sufficient number of shades evenly distributed between black and white.
A third solution resides in what can be called here the creation of a temporal grey scale in which the greys are obtained by separating each frame into at least two parts, namely one part during which a black state is written and another part during which a white state is written. Of course, the implementation of this solution produces a resultant grey owing to the retinal persistance of the information picked up by the eye. Unfortunately, this method requires screens addressed by extremely fast control circuits if it is desired to embody an acceptable number of greys. In fact, the number n of the sub-frames theoretically required to display 2.sup.m levels of grey is such that m&lt;n&lt;2 with, in practice, n barely differing from 2.sup.m. The preceding equality shows that, if it is desired to display via this method 16 levels of grey, that is if, m is equal to 4 with a video signal rate equal to that of television applications, namely 64 microseonds per line, it is only possible to have available 4 microseconds so as to display a line which is at the limit of feasibility.